Compound semiconductors include group III-V compound semiconductors such as gallium nitride (GaN), and group IV-IV compound semiconductors such as SiC. Switching elements that employ compound semiconductors include normally-on-type switches in which drain current flows when no negative gate voltage is applied, and in which drain current is interrupted when a negative gate voltage is applied.
In an inverting amplifier employing a field effect transistor (FET) with negative threshold voltage, an inverting amplifier circuit is formed as a series circuit from a FET with negative threshold voltage and a FET with positive threshold voltage. In the inverting amplifier circuit, the positive threshold voltage FET is connected to the low voltage side of the negative threshold voltage FET, and both FETs are switched ON by increasing the input voltages to the respective FET gates. By setting the input voltage to 0V, the positive threshold voltage FET is switched OFF. Configuration is thereby made such that the electrical potential of the source of the negative threshold voltage FET increases, and the negative threshold voltage FET is switched OFF, in accordance with a voltage equalizing resistor.
A compound semiconductor device is cascode-connected to a normally-on switching element and a normally-off switching element. In the compound semiconductor device the gate of the normally-on switching element is connected to the source of the normally-off switching element, and the compound semiconductor device is normally-off driven in accordance with a gate signal from the normally-off switching element.
The switching elements that use high voltage power source switching, such as those referred to as power transistors, are formed in a specific package. The leads formed in the package are connected for example to a board. There is significant parasitic inductance in wiring such as of the leads. Therefore, in the drive circuits of the normally-on switching elements, when the normally-off switching elements are off, an overpotential may be generated across the drain-source of the normally-off switching elements.
In the compound semiconductor device, across the drain-gate or across the drain-source of the normally-off switching elements, a voltage clamping mechanism is provided that uses a Zener diode or a capacitor.